The present invention relates broadly to an isolation amplifier apparatus, and in particular to the linear operation of power MOSFETs in a high voltage isolation amplifier apparatus.
Power MOSFETs are well suited for applications as high voltage amplifiers because of their ability to handle high peak power loads that are subjected only to average power limits, and also for their immunity to the second avalanche condition which is due high voltage transients. The linear operation of MOSFETs requires that the drive unit have a low impedance in order to control the gate voltage, because the input capacitance varies directly with the drain-source voltage.
The state of the art of isolated amplifiers and circuits is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 4,342,967 issued to Regan et al on Aug. 3, 1984; PA0 U.S. Pat. No. 4,441,068, issued to Smith on Apr. 3, 1984; PA0 U.S. Pat. No. 4,443,719 issued to Planar et al on Apr. 17, 1984; PA0 U.S. Pat. No. 4,455,526 issued to Miller on June 19, 1984; PA0 U.S. Pat. No. 4,461,966 issued to Hebenstreit July 24, 1984; PA0 U.S. Pat. No. 4,540,893 issued to Bloomer on Sept. 10, 1985.
The Smith reference discloses a circuit for driving an electrical motor which includes a linear transconductance amplifier 32 having an operational amplifier 68. The patented transconductance amplifier is provided with current sensing feedback.
The Hebenstreit reference in FIG. 5 shows a circuit for controlling a pair of power FETs 12 and 34 by way of a transformer 7 and transistor switches 10. The load is shown at 13 in this patent.
The Bloomer reference is concerned with a circuit for controlling a power FET with either load voltage or load current feedback.
The Planar et al reference discusses voltage isolation using a pulsed transformer. A high voltage, high frequency amplifier using FETs is taught in the Regan et al patent.
The Miller reference discloses high voltage, high power FET operation with transformer coupling and pulse width modulator in the control circuit. However, none of the references provide the overall combination of power FETs in conjunction with an isolation transconductance amplifier and a floating operational amplifier drive. The present invention is intended to satisfy that need.